A front page for The Onion dated November 22, 1963 reads: “Kennedy Slain By CIA, Mafia, Castro, LBJ, Freemasons.” I’d bet you a nickel that many people find that headline funny. I know it made me laugh. Although the assassination of John F. Kennedy is one of the most traumatic events in American history, the joke works because the reader is familiar with the barrage of wildly speculative and imaginative conspiracy theories that followed the tragedy, regarding the identity and motives of the killer. Yet most, if not all, of the parties mentioned above in jest have been proposed in all seriousness at some point as conspirators in Kennedy’s assassination in hundreds of books and documentaries. Such is the level of ridicule to which assassination theories have sunk in their efforts to seek closure to what is obviously still an unanswered question, and an open wound.

As the 50th anniversary of that fatidic day approaches, new and recycled conspiracy theories ­— along with corresponding anti-conspiracy rebuttals — are certain to hit the shelves and screens near you. In the hope that his ideas will receive attention as the commemorative feeding frenzy begins, G. Paul Chambers has released an expanded version of his book, Head Shot. In it, Chambers argues that speculating about the identity and motives of the killer, without trying first to develop a clear understanding of the means by which Kennedy was killed, is putting the cart in front of the horse. Before we can hope to answer who killed Kennedy, and why, we must know what killed him. (Yes, I said “what”). The impact of a bullet to his skull, you will say, was obviously what killed Kennedy. Agreed. But what kind of bullet was it? Of what caliber? Traveling at what speed? Coming from which direction? Entering the skull where? Hitting at which angle? Doing what once it hit? Causing what kind of wound? Chambers contends that such an understanding of the basic characteristics of the fatal projectile and the effect of its impact to Kennedy’s head must precede the questions about who fired it, which in turn should precede the questions about why he fired.

Although Kennedy suffered several serious wounds, there is no doubt that the mortal one was a head wound “too extensive to treat.” A part of his skull was actually blown off by the impact, a gruesome scene captured in 8mm film by a bystander, Abraham Zapruder. Shockingly, Kennedy’s autopsy was such a mess that there is actual disagreement between different sources regarding which part of the head was blown off by the shot. Thus, the head wound which appears on the side of the head in the Zapruder film, is later reported as being on the back of the head in a drawing from Kennedy’s examination in a Texas hospital, then as being in the top rear of his head at the official autopsy, and finally in the top front of the head in the official x-rays. Skeptical of the official x-rays and of the contradicting medical reports, Chambers resorts to the Zapruder film as his most trusted piece of evidence, since “films don’t lie.” Another thing that doesn’t lie is bone: a piece of skull was found in Dealey Plaza that day, not far from the spot where Kennedy was shot. Chambers states it came from the side of Kennedy’s head, making it consistent with the Zapruder film and providing information about where the bullet impacted the skull.

To determine the direction, speed and mass of the bullet, Chambers resorts to some basic knowledge of physics and ballistics. Since there was no exit wound on the other side of the skull, Chambers argues the bullet was likely a frangible, 0.223 small-caliber round, instead of a larger, full-metal jacket 0.26 round. By accounting for the fraction of momentum transferred to the piece of skull that was blown away and other brain matter that was expelled, Chambers is able to estimate the initial momentum of the bullet, which is found to be consistent with that of a small-caliber, high-speed round traveling at four thousand feet per second. The momentum is not consistent with a larger 0.26 round traveling at two thousand feet per second, which is the kind of round that can be fired by a Mannlicher-Carcano rifle.

Thus, Chambers claims, the most credible evidence available regarding Kennedy’s head wound, and its recoil at the time of the impact as captured in the Zapruder film, suggest that a rifle other than the one owned by Lee Harvey Oswald, possibly a Winchester .220 Swift, was used to fire the fatal bullet that ended Kennedy’s life. Furthermore, since Kennedy’s head recoils backwards in the film, the bullet must have come from the front. This direction is consistent with the grassy knoll, where many witnesses reported hearing shots and seeing smoke, and is not consistent with the “sniper’s nest” at the Texas School Book Depository. Chambers goes on to provide additional evidence in favor of the grassy knoll as a plausible firing place, from the statistical analysis of sounds and echoes recorded by police that day. He also mounts a frontal attack against the famous single-bullet theory, cornerstone of the single-shooter arguments, by means of two arguments: First, that the laws of physics preclude the sharp shifts in direction that are necessary for this theory to work, and second, that a pristine bullet that was recovered on a stretcher that day cannot be the “magic bullet” since the projectile that wounded Kennedy’s fellow passenger lost more mass than seems to have been lost by the retrieved bullet.

Although I could have done without the many extemporaneous references to science in general that pepper the text, Head Shot is a dense, fact-packed, and well-argued book. It casts powerful doubts on the official account of events and — through a seemingly sound thought process that makes use of familiar laws of physics — makes a good case that there were multiple shooters that somber day in Dealey Plaza. This automatically suggests a conspiracy. Chambers’ emphasis on working the question backwards using the fatal wound as a starting point strikes me as infinitely more compatible with scientific research than the approach taken by the Warren Commission, whose report was prepared from the start on the assumption that Oswald was the lone shooter. Also, by choosing his battles wisely, Chambers has produced a much more compact reading than most other recent tomes on the Kennedy assassination. Even though he stops short of even addressing the questions of who killed Kennedy and why, Chambers inquiry into what killed Kennedy should be taken seriously. If for nothing else, because it puts the horse back firmly in front of the cart.